The invention is directed to a device for and a method of preventing accidents due to tool jamming when working with hand-operated machine tools driving rotating tools, such as hammer drills, outfitted with an interrupter device for interrupting the driving action of a motor for the rotating tool depending on the operating conditions detected by a deflection sensor.
A serious problem in hand-operated machine tools, especially those with higher output, such as hammer drills, involves so-called rotation accidents, such as, injuries to the wrist or falls from ladders or scaffolding, and the like, as a result of the sudden jamming of the rotating tool and the consequent rapid increase in the reaction moment at the machine. Out of the large number of known solutions devised for this problem, reference is had to DE 44 26 305 A1 and DE 43 44 817 C2.
The first reference discloses a purely mechanical solution for this safety problem with an overload slip clutch, whose releasing moment occurs at a presettable adjusting speed depending on the occurring torque, wherein the clutch does not disengage during normal, trouble-free operation. If a rapid rise in the required torque should occur in the event of malfunction, for example, when the tool is jammed, the slip clutch, which is affected by inertia or damped by volume displacement, is released and immediately interrupts the line of transmission of force from the motor of the device to the rotating tool.
The second reference discloses a mechanical-electrical solution. In this reference, an anticipated deflection angle is calculated by means of an acceleration pickup. If this deflection angle is greater than a maximum presettable deflection angle, an interrupter device, in particular an electromagnetic clutch, is released before the maximum deflection angle is reached, possibly in combination with a shutting off of power. The acceleration pickup is an angular acceleration sensor whose measurement values are converted to the expected rotational angle by double integration over the duration of a time constant.
The first solution presents difficulties in that an untimely releasing of the safety clutch can also occur as a result of intended manipulation of the machine, such as when working with a hammer drill in a concrete mass of inhomogeneous composition. This has to do with the use of passive signal evaluation, particularly because the movement sensor and actuator are comprised in one unit and purely mechanical principles are applied.
Difficulties arise in the second solution because this model-based solution is founded on a weighted addition of the instantaneous angle calculated from the measured angular acceleration with the predicted amounts of angular velocity and the angular acceleration relative to the angle occurring after the expiration of a forecast time, wherein an adjustment of the system parameters must be carried out empirically based on difficult-to-identify parameters of the process model. In this case, it is hard to modify the process model or adapt it to different machines. Further, once the process model has been set, the summation can not be weighted differently in this procedure. Thus, it may come about, for example, that the interrupter device is activated when only one of the input variables, for instance, only acceleration, is very high, although there is no risk of an accident.